Luxrender - How to select API 2.x?
I'm just setting up a Mac and aren't too familiar but having just downloaded the latest LuxRender1.5 and installed LuxBlend, I see the API choice is not called API2.x, rather it is called LuxCore API (experimental) so I am assuming this is the 2.x
Here is a screenshot:


EDIT: But be aware that there may be performance issues when using the viewport. This post mentions and gives some explanation:
https://groups.google.com/forum/#!topic/appleseed-dev/YZ5dR9qvUV0